Porcelain enamel



July 20; 1943 M. .1. BAI-IINsEN ETAL `2,324,812

PORCELAIN ENAMEL 2 sheets-sheet 2 Filed July 25, 1941 5 lo CALCIUM FLUGRIDE- INVENTOR5 0A/R05 J. eAHA/.sf/y ma BY Evan/ E. 32m@ ATTORA/Ey Patented July 2l), 1943 PORCELAIN ENAMEL Monroe J. Bahnsen, Lakewood, and Eugene E. Bryant, Bedford, Ohio, assignors to Ferro Enamel Corporation, Cleveland, Ohio, a corporation of Ohio Application July 25, 1941 serai No. 404,048

' s claims. '(01. 10s- 48) This invention relates as indicated, to porcelain enamel, and more particularly to procelain enamels characterized by the combined inclusion therein of zirconium oxide and phosphorus.

This application is a continuation in part of our co-pending applications Serial No. 311,362, filed December 28, 1939, now Patent No. 2,250,456, and Serial No. 361,448, filed October 16, 1940. now

I IPatent No. 2,250,457.

From an inspection of our aforesaid co-pending applications it will be observed that we have disclosed and claimed .therein the combined use in antimony-free porcelain enamels of zirconium oxide within a rather narrow range and phosphorus in the form of P205, likewise in a relatively narrow, although lower, range. Our investigations as disclosed and claimed in Vour aforesaid co-pending applications therefore extended to the discovery of an unexpected result produced -by the combined presence within said percentage ranges of zirconium oxide and phosphorus in antimony-free enamels.

In the present application we shall give the results of considerable additional development work which has shown that when certain other factors are controlled in the manner hereinafter specified, this cooperative relationship between zirconia and phosphorus' may be utilized in a much wider range of enamels than those disclosed in our said colpending applications.

Our further development work, as-reported in this continuation in part application, includes within its scope all of the .work reported in our previous applications as well as a large amount of additional work, from a careful study of which it will be observed` that this wider range within which the cooperative effect between zirconia and phosphorus is demonstrated is determined by certain balances between the constituents such as the monovalent basic oxides such as'sodium and potassium oxide.

It is a principal object of our invention therefore to provide a wide range of enamel compositions within which the' cooperative effect of zirconia and phosphorus may be utilized in the production of `enamels which have superior properties, especially from the standpoint of opacity,

Other objects of this invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, said invention, then, consists of the means hereinafter fully described and particu-v larly pointed out in the claims.

The following description sets forth in detail certain approved combinations of ingredients embodying our invention, such disclosed means constituting, however, but representative examples in which the principles of the invention may be used. 'Ihe several drawings comprise charts l showing the effect upon the composition of the variance therein of certain basic components of the composition. v

In said annexed drawings:

Fig. 1 is a chart showing the effect on the percentage of reflectance, i. e., the effect on opacity, of a representative enamel of our invention by varying the zirconia content thereof;

` Fig. 2 is a chart similar to Fig. 1 showing the effect of varying the zirconia in an enamel composition generally similar to Fig. 1 but in which the sodium fiuosilicate component of the composition of Fig. 1 is replaced by cryolite;

Fig. 3 is a chart similar to Fig. 1, utilizing however a somewhat different basic formula;

Fig. 4 is a chart similar to Fig. 3, showing the effect on opacity of variations in the amount of zirconia in a composition generally similar to that shown in Fig. 3, but in which the sodium fiuosilicate Iof the composition shown by Fig. 3 has been replaced by cryolite; and v Figs. 5 to 16 are charts/which show the effect on reflectance of opacity of variations in the concentration of other specifically identified com# ponents of the basic formula of Fig. 2, the same amount vby Weight of all constituents, excepting the one varied, being used for all of the compositions of Figs. 5 to 16, it being observed that when the same Weight of each of the components excepting the one varied is maintained constant the percentage of the constant components changes slightly as their total percentages of the composition varies in accordance with variations in the variable.

For the reasons which will be explained later It will be observed that a number of components have been identified as ranging from zero to a. maximum stated amount. This means that satisfactory enamels may be made by omitting completely the particular component, and again certain components may be omitted entirely only if a substitute therefor, also included within the tion of such proportional relationship as should be maintained between the various components mal manner as used by any Chemist 01' l5 of the composition for best results.

Before proceeding with such explanation, however, it is deemed advisable to rst include the following Table II which gives a range of possible raw batch compositions from whichf may be y be the Chemical smelted enamels having the calculated oxide conequivalent of the oxide and the fillO tent included within the scope of our invention as given in Table I.

Timm: IIl

Per cent up to 17.98 9.83 up to 26.40 6.15 up to 24.92

and in greater detail, the compositions of our invention can best be identified by having reference to the oxide content of the resultant enamel produced by smelting the raw batch mixture. Throughout the following description the oxide content of the smelt has been given based on a theoretical calculation of the raw materials which enter into the smelt. At this point we should extable of compositions, is included in the enamel.' plain that these theoretical or computed oxide We shall hereinbelow refer specifically to each compositions have been arrived at in the follow- 10 of the components mentioned in the foregoing ing manner: Table I for a better understanding of the ranges In all cases except the llOlideS. the ralW magiven in such table and for a further explanarials consist essentially of oxides or combinations of oxides and these materials are broken down in the nor ceramist, the volatile ingredients of these components are considered as lost, leaving the single oxide content. However, in case of fluorides, for the sake of simplicity,y the metal ion or ions in these iluorides are calculated to rine is calculated as molecular fluorine, assuming no loss thereof. While there may be some loss of fluorine during the smelting operation, the amount is generally indeterminate, and while other workers in the art may 'employ a slightlyv different mode of actual up to 16.25 up to 5.94 up to 6.76 up to 4.22 up to 0.81 up to' .094 up to 12 up to 4 up to 16 up to 9 up to 8 up to 8 up to 2 up to 099 up t0 1.23 3 54 up to 13.52

examples speciilc enamels which have been produced and which are Itis also believed desirable to set forth hereinbelow in Table III a sufllciently large number of Feldspar--- Zircon sand 16.42 up to 29.58 t forth the Sodium nitrate F1uorspar Sodium iiiuosilicate Bone ash Aluminum hydrate Zinc oxide 'Iitanium dioxide -Manganese dioxide Boric acid 40 Calcium carbonate Cryolite Kaolin--- Magnesium carbonate. Phosphoric' acid Barium carbonate Sodium carbonate representative identified` both by their raw batch as well as calculated oxide composition.

37 7o 7o s2 Silica sand Dehydrated borax TABLE III Raw mirc tice, whatever mode of computation is used, to assume no loss of iiuorine.

up to 12.00 5.50 up to 12.00 0 up to 4.15

up to 1.00

u p to .1.00

up to 2.00

up to 4 50 up to 5.00

lculated oxide y Per cent S1011- 23.00 up to 35.00 ZrO2 10.00 up to 20.00 B11Oa 10.00 up to 20.00 NazO. up to 14.25 up to 14.25

5.00 up to 13.00 1.00 up to '7.50 0 0 0 0 0 Taal.: If

In the following Table I we have se ranges of percentages of the calculated oxide the enamels whichvcomprise our inA {66nate.... Phosphoric acid.

of calculation in determining the ca content, it is nevertheless general prac odium iluosilica Boncash Aluminum hydrate Barium carbonate...

Sodium carbonate Calcium antimonate content of vention K10- C8O A1203- ZnO- Fz P205- 'r1o2 M1102- BaO MgO.. 'SbzOsn-n Silica sar\d Dehydrated borax Feldspar -Zircon-sand.

Sodium nitrate,

Fluorspar S Calcium carbonate Cryolte Kaolin Magnesium ca The effect of varying alumixla. (A1203) is sho in Fig. 9 vand from the curve there given, as well as from actual observation of the enamels Aproduced, it may be noted thatwhen the alumina is dropped to very much below the low range specified the enamel has a tendency to tear, and as the upper end of the range is approached or passed the enamel tends to produce a matte finish. The range specified in Table I is the range within which best reflectance was observed. Specific example 21 is representativey of the enamel having an alumina content near the low end of the range and specific example 22 is representative of the upper end of such a range.

It is economically desirable to omit zinc oxide from enamels if at all possible. The addition of zinc oxide must therefore be justified, and while the addition of up to about 4% has produced a slight increase in opacity as is shown by Fig. 10-

with good surfaces maintained throughout the range, the enamels in the low zinc oxide range will be found to givebest results when the zinc oxide is replaced with an equal weight of alumina (A1203).

Example 23 of Table III is an example of an enamel which contains no zinc oxide and specific example 24 shows an enamel containing zinc oxide near the upper end of the range.

The effect of varying titanium dioxide (TiOz) has not been shown by any chart. Titanium dioxide is added to this class of enamels to control color when desired. It is not necessary in most enamels of this class and it will be noted that when too high a concentration is used the enamel i has a tendency to be yellowish.

The same comments made in connection Vwith titanium dioxide apply equally to manganese oxide. However, when too large an amount of manganese oxide is used the enamel turns pink.

Barium oxide may be used in enamels of this character for the purpose of softening the same if this should be desired. By referring to Figs. 13 and 14 it will be noted that as the barium oxide is increased opacity falls off sharply. Fig. 14 shows the eiect of barium oxide in replacing calcium oxide in these glasses. The solid line shows a weight for weight replacement of barium oxide forvcalcium oxide, and it will be noted that the reflectance drops ofi' rapidly when more than 2% barium oxide has been added. The'dotted lines of the same gure shows the effect of molecular replacement with barium oxide for calcium oxide and the drop in reflectance is even more rapid than in the previous case. Fig. 13 shows the direct addition of barium oxide to enamels of this type and it is observed that the reflectance is lowered tremendously. This would preclude the use of barium oxile except when it is used as a replacement material for calcium oxide. Specific examples 29, 30 and 31 are enamels which show these three types of substitution.l

Fig. 16 should be referred to in studying the effect of the addition of magnesium oxide. Magnesium oxide should be expected to increase opacity. However, when added to the enamels in the form of magnesium carbonate (MgCOs) a tendency to produce matte finishes results. Fig. 16 shows the reflectance change caused by the addition of magnesium oxide. The dotted line shows direct addition of magnesium oxide to this type of enamel (added as magnesium carbonate). It is seen that the reflectance increases slightly with this addition. The solid line which shows less increase in reflectance is based upon a molecular substitution of magnesium oxide for calcium oxide. Table I II may be referred to for enamels containing in both cases magnesium oxide near the upper limit of the range given.

No chart has been drawn showing the effect of the addition of various amounts of antimony oxide on the opacity of the enamels of the character to which this invention relates. However, it has now been found that amounts up to 5% of antimony oxide may be added (see specific ex ample No. 36 for an enamel of this character) without impairment of the reflectance. Ordinarily antimony need not be added as an intentional raw batch constituent. However, the importance of the data on this material relates to the possible inclusion of antimony in cases where reclaimed enamel is melted as a part of the raw batch composition. In other words, it has been found that the enamels of our invention can tolerate up to as high as 5% of antimony when it thus finds its way into the smelt. In studying the effect ofthe presence of antimony, i. e., in

producing specic example No. 36, antimony was intentionally added to the smelter in the form of calcium antimonate. It should be observed, however, that when antimony is present in the composition the sodium content of the enamel should be maintained as low as possible within the ranges above given.

No chart has been given for the effect of variations on the opacity by variations in the amount of fluorine present in the smelt. The low fluorine enamels were obtained by use of lower amounts of cryolite and can thus be more clearly discussed later under this raw material. The normal iluorine range for enamels of this type, from about 5% to about 13%, can be obtained by the use of either fluorspar, sodium fiuosilicate or sodium aluminum fluoride (cryolite) 'I'he two latter raw materials are more or less interchangeable by making correct molecular substitutes. 'I'he normal enamels of this series will in general contain iluorine near the upper end of this range.

The main function of fluorine (fiuorides) is to act as a flux and produce enamels of low enough melting temperature to be useful commercially. According to some authorities, iluorine also affects the opacity and it is generally thought that the higher'the fluoride content the higher the opacity.

For the effect on reflectance or opacity of various amounts of calcium oxide, reference may be had to Fig. 11. The curve of Fig. 11 shows that as the `calcium oxide content is increased from zero there is a more or less progressive increase in the amount of reflectance in the enamel until the upper range of 12%, given in Table I, is reached. Amounts of calcium oxide above 12% produce no appreciable increase in the amount of reflectance and are further accompanied by a hardening ofthe glass. It may be observed that as the upper end of the range for calcium oxide is approached the' enamel has a tendency to produce a matte surface. Reference may be had to specific examples Nos. 25 and 28 which vare respectively examples near the low and high vof varying the amount of sodium is strikingly illustrated in Fig. 6. Sodium is present in most commercial enamels of the type to which this invention relates because sodium borate (borax) is Specific examples 34 and 35 of'- the normal source of the B203 radicle, also soda and potash both are contained in feldspar which is one of the most generally used commercial sources of alumina. Sodium nitrate is also selected as an oxidizing material in the enamel batches because of its cheapness, and the two iluorldes which are used to contribute most of the fluorine to this type of enamel, namely cryolite and sodium fluosilicate,4 both contain high percentages of sodium oxide. For these reasons no dat-a on commercial glasses with less than about 8% sodium oxide are given in Fig. 6, and such amount .represents the minimum necessary in order to include in the composition 4the material with which it is combined in the rawmaterials available. The curve above about 8% that if the soda content of the composition is shows, however,.

example l, is based on enamels containing sodium fluosilicate. Fig. 2, as previously indiv cated, is based on acomparable composition (see specific example 8) 'in which sodium fluosilicate' I was substituted by cryolite. Similarly, the chart of Fig. 3 is based on an enamel containing soraised intentionally or by lack of careful control or to reduce it to a very small percentage, this would be a desirable improvement. The range of soda has, therefore, been given as extending to not morel than about 14%.

l apply also to potassium oxide when the two elements are treated on a molecular equivalency basis. Thus it is highly desirable to keep the mono-basc metal oxide content of the enamel l as low as possible, that is, as lowas feasible in view of commercial considerations.

, As previously indicated, the enamels of the present invention are all characterized by the `.effect between ZrO2 and phosphorus which is `conveniently present in the form of P205. As i; likewise previously indicated, the chart of Fig. 1 shows the eiect of varying the zirconia content of an enamel which has a basic composition f similar to specific example 1. In other words, `the various points which were determined and `which xed the curve of Fig. 1 were arrived at by holding constantthe values for all of the other components of specificv example -1 and varying only the amount of zirconia. This was accomplished by varying the amount of zircon sand and at the same time varying the amount of silica sand so as to maintain in the composition jl asubstantially constant amount of silica. Fig. 3 `similarly shows the effect of variations in the i amount of zirconia in the basic formula as represented by specic example No. 4, and in arriving at the various points which determine the curve of Fig. 3 all of the raw batch components with the exception of Zircon sand and silica were `held constant. Zircon sand was varied to yield various amounts of zirconia, and the silica sand was varied so as to maintain a` substantial amount lof silica in the composition. The results given "in Figs. 1 and 3 have been verified by other data, `i'iot shown, in which the raw batch constituents ifwere all held constant with the exception of lto the smelter. Inasmuch as zirconia is availiable in the enamel more economically as Zircon `fsand, it was felt desirable to base the data herein on variations in the amount'of zircon sand.

The comments made above with respect to soda` combined presence therein and the cooperative dium iluosilicate (see specific example 4) and4 for a `comparison Fig. 4 is based on an enamel (see specific example 11) in which sodium liucsilicate is-substituted by cryolite. The effect of zirconia as an opacier, both in the form of a smelter addition as well as a mill addition; has been under investigation by workers in the art- Hartmann thirty years ago exy for many years. plored the use of zirconia in concentrations up to more than 20% as an opacier in conventional enamels. 'I'he use of zirconia by itself has, however, been accompanied by certain difliculties, which difficulties are becoming increasingly acute according to the increasingly rigid requirement to which porcelain enamels are subjected.

While a zirconia opacifled enamel as produced by Hartmann was satisfactory .in meeting the requirements of enamels thirty years ago, such requirements can no longer be met by the use of zirconia'alone. It may be observed that about 1910 Hartmann proposed"`enamels containing approximately 15% zirconia (see Grunwalds The Raw Materials for the Enamel Industry and Their Chemical Technology, Charles Griilin 8a Co. Ltd., London, 1914, page 129'), and-other workers in the art (see, .for example Kinzie Patents Nos. 1,848,567 and 1,944,936), have pro-l posed using zirconia in amounts of about 15%... These enamels judged by present day rigid standards, however,` are not entirely satisfactory and our invention is therefore concerned with `the combined use with such zirconia bearing enamels of substantial amounts of phosphorus which is conveniently present in the composition in the form of P205.

By having reference to Fig. 5, the powerful cooperative eiect of P205 on the opacity of the enamel will be observed. At this point it may be well to mention that all of the other enamels, that is, the enamels on which the other charts of the several figures were based, all contained the zirconia phosphorus combination.

This was felt desirable because it is only possible to secure consistent results in zirconia bearing enamels when the zirconia phosphorus combination is used.`

At this point we should explain that the curves of Figs. 5 to 16 were all determined by first selecting a particular enamel having a composition of specific example 8 of Table III and varyfing only that component the effect of whose variation on the opacity is shown in a particularl curve. This variation was achieved variously by a change in those raw batchcomponents which contribute the Variable to the smelt. It is believed that from an inspection of the various specific examples given it can readily be deter-` mined by thoseskilled in the art the manner in which variations were made in the raw batch composition in .order to effect the required variation in the calculated oxide content in order to produce the variations represented by the several curves. In other words, specific examples 1 to are representative of a vary large number of smelts which were made up in determining the points on the various charts or figures which determine the curves thereon. Specific examples 1 to 12 represent various smelts in which changes were made in the zirconia content and specific examples 13 to 35 represent smelts made holdingthe zirconia substantially constant and varying certain other components. Attention is directed to the bottom of Table III where a series' of figures are given for each of specific examples 1 to 14 indicating the proportional relationship between the amounts of P205, sodium oxide and zirconia present in the calculated oxide composition of such specic examples. It was felt unnecessary to include in the table the proportional relationship which exists between these three critical elements in the remaining specific examples.- That information can be determined readily from the values given in the table.

The reflectance values for the 36 specific examples given in Table III are available but it is believed that such information instead of being included in Table III can be much more intelligently studied if reference is had to the several charts in the drawings. In other words, the reflectance values if entered on Table III would give` only single points on the curve whereas by having reference to vthe charts themselves the` full picture of the eifect on reectance of the particular component may be more readily determined.

In Table I certain components have been indicated as being permissibly absent. It is believed that the previous discussion of the effect on refiectance of variations in each of such components has explained the reason why Table I indicates the permissible absence of certain of such components.

Now with respect to Table II, it will be observed that the same indicates the permissible absence of a rather large percentage of the available raw materials, and with respect to that table the following comments may be observed.

Silica sand may be omitted entirely if there is included in the lcomposition other silica bearing materials which will supply in the calculated oxide content of the resultant smelt the 23% since phosphorus will not occur'in its pure state f minimum of Table I. Sources of silica may be Y zircon sand, sodium fluosilicate, kaolin and feldspar. This manner of varying the raw batch y mixtures in making available la particular component of the` calculated oxide content from various sources is common practice and well known to those familiar with the art.

While zircon sand is specifically mentioned as one of the raw materials, it is Well known that other sources of zirconia may be substituted. for the zircon sand. Such possible sources include pure zirconia and baddeleyite, a crude zirconium oxide which may soon become available in this country in commercial quantities. Sodium zirconium silicate and other compounds of a similar nature may also be substituted in place of the Zircon sand.

The partial or complete substitution one for the other of sodium fluosilicate and cryolite has already been explained.

Bone ash is used as the source of P205 in the smelt. At this point it may be well to state that throughout this specification where reference has been made to phosphorus, it is of course understood that reference to the oxide was intended,

in the smelt. Insteadl of bone' ash anyfother available source of phosphorus may be used, even to the point of using pure phosphoric acid, although phosphoric acid will be found unveconomical in view of the more economically available raw materials like bone ash which can contribute phosphorus.

In adding alumina to these enamels, feldspar is generally considered as the principal source, and the permissible maximum of feldspar is determined by the alkali content thereof. After as much feldspar as is possible has been added, the remainder ofthe alumina is added in the form of kaolinunless the total amount of kaolin required to Acontribute the remaining alumina would raise the silica content too high, in which latter event the necessary deiiciency in alumina can be contributed by the addition of aluminum hydrate. Since cryolite is also a source of alumina, those enamels which employ cryolite may not require the addition of any aluminum hy-- drate in order to supply the required minimum of A: in the calculated oxide content as given in Table I. l

, The total'V omission of 'zinc oxide from the enamel and its replacement with an equal weight of alumina has already been discussed so that further reference to the omission of zinc oxide is believed unnecessary. The same is true'with respect to titanium dioxide and manganese dioxide.

Boric acid is quite expensive and need be added only to soften the enamel in cases of hard enamel which require to be softened and which hard enamel already contains the maximum permissible amount of sodium which is usually/relied upon to provide the softening effect for which boric,acid is used.

Calcium carbonate is used only as an auxiliary source of calcium Where the other raw materials added to the smelt are not suiiiciently high in calcium to provide that amount desired in the enamel. Since Table I shows that calcium oxide may be omitted entirely from the enamel, it is obvious that calcium carbonate may likewise be omitted entirely from the raw batch mixture.

Kaolin is, as previously indicated, employed as an auxiliary source of alumina to the extent that the maximum silica content of the smelt permits. If the other alumina containing raw materials provide sufficient of that component, then kaolin is not required. i

Magnesium carbonate, barium carbonate and sodium carbonate are added only if for some reason the corresponding oxide should be desired. Reference to the discussion of the effect of such corresponding oxides, previously given, may be had for a full understanding of the reasons why these three carbonates may be omitted entirely from the raw batch mixture.

Phosphoric acid is mentioned as a possible raw batch constituent. However, as previously indicated, since bone ash is a more economical source of phosphorus, the latter will usually be employed and phosphoric acid omitted entirely.

Fig. 12 shows the important effect of iiuorspar (CaFz) on the reflectance of this type of enamel. Fluorspar is used as the basic source of fluorldes l and as much of it as possible is usually employed 11, which refers to the effect of variable calcium oxide content, the importance of fluorine in producing opacity is quite obvious.

Reference to Fig. 15, which shows the effect of variable sodium aluminum fluoride or lcryolite on reectance, is of interest because of the defl- `n1te peak that is obtained. At less than 2% cryolite the reflectance is low, undoubtedly due to the low iiuorine content of the enamel. From enamels of excellent surface are obtained. When the cryolite percentage becomes greater than 14% the reflectance is appreciably lower and this is undoubtedly caused by the fact that while we are adding 'appreciable quantities of iluorine we i are also adding large quantities of sodium oxide which has astronger deopacifying effect than iiuorine has an opacifying eiect. All of the charts, i. e. Figs. 5 to 16, have, as A previously indicated, been based on enamels which have the basic formula of specic Example 8 and are characterized by the combined `presence within the desirable proportional relationshipof this invention of phosphorus and f `zirconia. Other data, which it is believed unnecessary to include in this description, shows that the various curves which have been given for modifications in the specic composition of Example 8 are representative of results which `would be secured from similar modifications in l other compositions falling within the broad range of our invention. A considerable number `of smelts have, been made and .the reiiectances l thereof determined where the phosphorus component was omitted and zirconia used without Such enamels have two pronounced characteristics different from those of our invention. In the first place, the auxiliary effect of phosphorus definitely increases the opacifyingfeffect of zirconia with which it is combinedand accordingly *the phosphorus containing enamels have a unij formly higher reflectance than those enamels 1 containing the same amount of zirconia and from which the phosphorus has been omitted. The second principal characteristic of the phosphorusree zirconia bearing enamels is their lack of uniformity with respect to reflectance which may lfbe due in a measure to their apparent extreme sensitivity to the conditions under Which they are smelted. An attempt was made to correlate the data obtained on a large number of smelts `of phosphorus-free zirconia containing enamels in order to show a curve indicative of the variation in reflectance due to variations in the zirconia content.` All of these smelts were made under most carefully controlled conditions by the if same worker and in the same equipment. As a fmatter of fact, the same careful control of con- :ditions in the manufacture of these phosphorusfree enamels was observed as in the phosphorus Icontaining enamels as to which the reflectance curves are given herein. A variation of not more l"than 4%in reflectance is permitted between sep- `:irate smelts if they are to be considered as check "`\resu1ts. In the preparation of thel very large number of smelts of the phosphorus containing I"enamels which were prepared in compiling the data given by the curves of the figures herein, as many as twenty-six smelts of the same composition were manufactured and in those twentyance was within the 4%,limit above given. Howover, in the manufacture under identical condiretlectance of the phosphorus-free enamels varied V 2% to 12% cryolite the reflectance is high and l `the auxiliary effect thereon of the phosphorus.

six smelts the total variation in observed reectof zirconia in theabsence of phosphorus.

so widely that it was impossible` to draw a single curve representative of the reilectances which might be secured over a range of concentration In connection with this comparative work, two series of smelts under identical conditions in the same apparatus by the same worker showed differences in observed reflectance of as great as 10%. For this reason no attempt has been made to include in the figures a curve which is representative of the eiect of zirconia in varying proportions when used without the auxiliary amount of phosphorus.

Zirconium enamels generally, of which those of our present invention are representative, generally have the characteristic of developing opacity by the formation of a crystalline phase in a glass matrix, which crystalline phase has a different index of refraction than the matrix, and accordingly impart opacity thereto. As smelted. these enamels are relatively clear and the development of this crystalline phase occurs during the fusion of the enamel on the work. We

have now discovered that by annealing the enamelr after smelting and while it is in a semi-J viscous or solid state and prior to its application to the work the ultimate opacity developed by the enamel during the fusion thereof to the work may be materially increased. Wel believe that this discovery that the ultimate reflectance or opacity of the enamel may be thus increased by annealing the same at a temperature where the,

enamel is not flowable, is new withus. The enamel, after smelting, may be annealed prior to fritting, or the conventional frit may be annealed. We have successfully annealed enamels of this character in a temperature range of from about 1l00 F. to about 1400 F. intime intervals which vary largely dueto the condition ofthe enamel glass prior to heating.` If the enamel glass is in the form of cold frit, a longer time will be required than when the annealing period starts upon a reduction in, temperature of the enamel glass from a higher temperature to the annealing temperature range.

The annealing of the enamel glass, as indicated, makes possible the attainment of a higher ultimate reflectance in the enamel as fused on the work than when such annealing step is not employed. Accordingly, by the employment of such annealing step the same ultimate reflectance may be achieved by the use of cheaper materials, thinner weights of application and lesser amounts of opacifier mill additions. `It is also possible to achieve the desired reflectance in furnaces which fuse the enamel on the work in such a short length of time that without preannealing the necessary opacity could not be obtained. i

It is within the contemplation of our invention to pour the enamel glass from the smelter and instead of immediately fritting the same by projecting the stream into a water bath, to roll the stream into a thin sheet much as in the process used for the manufacture of sheet glass. The rolled sheet is then passed through a furnace or annealing chamber where it may be maintained at an annealing temperature for a period which may vary from about 1/2 to 5 minutes. After the sheet issues from the annealing chamber it may be conveniently broken up by the projection thereagainst of a high velocity stream of water or steam or Aby the immediate projections and with the same degree of control the l tion of the sheet into a cold water bath. When a high velocity stream of water is employed as the fritting means, and particularly when such and at the same time suillciently cool so thatit is in a non-plastic state.

Other modes 'of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

We, therefore, particularly point out and distinctly claim as our invention:

1. The method of making a porcelain enamelled article having an opaque coating Whichcomprises smelting a raw batch mixture of porcelain enamel forming ingredients characterized by being able to yield from the smelt a frit containing from about 10% to about 20% ZrO2 and from about 1.00% to about 7.50% of P205 based on the total weight of: the frlt, milling a slip containing said frit, coating the ware with said slip, and burning the same to yield said opaque surface.

2. An opaque vitreous enamel characterized by high reflectance and containing about:

5. An opaque vitreous enamel characterized by high reflectance and having a mono-basic metal oxide content of not more than about 14.00%

Per cent Zr02 From 10.00 to 20.00 P205 From 1.00 to '1.50

based on the total weight of the enamel.

3. An opaque vitreous enamel characterized by high reflectance and having a mono-basic metal oxide content based on the total weight of the enamel of not more than about 14.00% and containing about:

I Per cent Zr02 From 10.00 to 20.00 P205 From 1.00 to 7.50

based on the total weight of the enamel.

4. An opaque vitreous enamel .characterized by high reflectance and containing about:

based on the total `weight of the enamel.

based on the total weight of the enamel and containing about:

based on the total weight of the enamel.:

6. An opaque vitreous enamel of high reflectance characterized by the presence of about:

Per cent Zr02 From 10.00 to 20.00 P205 From 1.00 t0 7.50

based on the total weight of the enamel and a mono-basic metal oxide content of approximately the same as the ZrO2 content but not substantially in excess'of about 14.00% based on the total weight of the enamel.

7. Anopaque vitreous enamel o1' high reflectance characterized by the presence of about:

. Per cent ZrOz From 10.00 to 20.00

P205 From 1.00 to 7.50 g

based on the total weight c! the enamel and a P205 1 ZlOz A 5 Mono-basic metal oxide 5 0 based on the total weight of the enamel.

8. An opaque vitreous enamel of high reflectance comprising based on the total weight of the enamel from about 10% to about 20% ZrO2 and P205 in an amount equal to about 20% of the amount of ZrOz present.

MONROE J. BAHNSEN. EUGENE E. BRYANT. 

